1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to light emitting devices having an ohmic contact formed on an epitaxial layer.
2. Description of the Related Art
Light emitting diodes (or LEDs) are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes generally comprise a p-n junction formed in an epitaxial layer deposited on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that comprise the active region of the device.
Commonly, an LED includes an n-type substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region. In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact must be formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
Because it is difficult to make highly conductive p-type Group Ill-nitride, materials (such as GaN, AlGaN, InGaN, AlInGaN, and AlInN), lack of current spreading in the p-type layer may be a limiting factor in the performance of LEDs formed from such materials. Accordingly, it is desirable to form an ohmic contact over as much of the surface area of the exposed p-type layer as possible in order to induce current to pass through as much of the active region of the device as possible. Moreover, patterning an ohmic contact conventionally requires at least one photolithography step. Photolithography may be an expensive and time consuming step in the semiconductor device fabrication process. It is desirable to reduce the number of photolithography steps required for device fabrication.